A placeholder in a form _n is simply a synonym for the corresponding
arg<n> specialization. The unnamed placeholder _ (underscore) carries
special meaning in bind and lambda expressions, and does not have
defined semantics outside of these contexts.

Placeholder names can be made available in the user namespace through
usingnamespacempl::placeholders; directive.

[Note: The include might be omitted when using placeholders to construct a Lambda Expression for passing it to MPL's own algorithm or metafunction: any library
component that is documented to accept a lambda expression makes the placeholders
implicitly available for the user code — end note]